1. Field of the Invention
This invention relates to improved power and refrigeration thermodynamic systems which include as a cycle or subcycle within the systems, a hydride-dehydride-hydrogen cycle yielding, at one phase of the cycle, relatively high pressure hydrogen gas at an elevated temperature.
2. Brief Description of the Prior Art
In our copending U.S. patent application Ser. No. 553,248, filed on Feb. 26, 1975, now U.S. Pat. No. 3,943,719 we have described the continuous development of power and refrigeration in an efficient manner, utilizing a hydride-dehydride-hydrogen (HDH) cycle. For continuously supplying relatively high pressure hydrogen gas, a plurality of hydride-dehydride reactors are provided and are operated in out-of-phase or staggered sequence so that during the period when low pressure, relatively cool hydrogen gas is being charged to one of the reactors, another is being activated and another being dehydrided to produce high pressure hydrogen gas. The pressure energy of the gas thus developed in the hydride reactors is used for continuously developing power and refrigeration, following which the hydrogen gas, at reduced energy, is recycled to the reactors to recommence the HDH cycle. In order to chemically compress the hydrogen gas in the form of its hydride, a low grade thermal source is utilized to supply heat to the several reactors.
In one aspect of the use of the HDH cycle as described in our copending application, the compressed and heated hydrogen gas which is released during the dehydriding phase of the HDH cycle is either passed directly to an expansion device, such as a turbine, or is utilized for transferring heat via a heat exchanger to a secondary or auxiliary system in which a heat input is desired. It is also contemplated, in the use of the HDH cycle as described in our copending application that the cold exhaust from the power generating expansion device can be used in a heat exchanger to provide refrigeration prior to recharging the depressurized hydrogen to the HDH reactor bank.
The described methods of utilization of the high pressure heated hydrogen gas which is developed as a gaseous product discharged from the reactor bank in the course of carrying out the continuously operated HDH cycle represent but a few of the uses which can be made of the hydrogen gas in its forms and energy states during the transition occurring between the time of discharge from the reactor bank in a pressurized state, until recharging to the reactors to recommence the hydriding process.
The Carnot cycle defines the limit of thermal efficiency which can be realized in the use of any heat engine operating in a cyclic manner. In actual practice, one approach to Carnot cycle ideality has been through the use of a power cycle referred to as the Rankine cycle. In the Rankine cycle, reversible adiabatic compression is followed by constant pressure heat transfer for heating, reversible adiabatic expansion and constant pressure heat transfer for cooling. Since it is easier to pump a liquid than a mixture of liquid and vapor, the Rankine cycle condenses the vapor and uses a pump to reversibly and adiabatically compress the liquid. Moreover, constant pressure input to the boiler is employed.
Other practical variations can be used to even more nearly approach the ideal Carnot efficiency in utilizing the Rankine cycle. Thus, by lowering the exhaust pressure from a turbine expander used in the cycle, more work is taken out of the compressed fluid passed through the turbine, and there is less rejected heat. This results, however, in a large moisture content in the exhaust from the expander. Also, superheating of the fluid charged to the expander allows an increase in efficiency plus the added benefit of raising the quality of the steam in the exhaust. All of the described improvements are in the nature of increasing the inlet pressure and/or temperature to the turbine expander and/or lowering the exhaust pressure and/or temperature. Rejected heat, while utilized to whatever advantage it can be used so as to approach Carnot efficiency, is nevertheless a secondary consideration, since present day Rankine cycle plants have been most economically designed to produce the most power. It continues to be of importance to consider ancillary equipment that can make better and more efficient usage of the rejected heat.
The maximum thermal efficiency of all power plants, whether using the Rankine cycle, the Brayton cycle or other power cycle, have been practically evaluated for many years. Little attention has been directed to the other end of the thermal energy spectrum--i.e., the lower limits for heat rejection. In most textbook considerations of this aspect of power cycles, the subject is dealt with as if such lower limit were near ambient conditions applicable to the power cycle, and in general this is about 289.degree. K. Thus, most energy availability evaluations are based on approximately this temperature. Among the more promising utilizations of rejected thermal energy which have been proposed to this date are the use of this energy to heat buildings or to heat ponds utilized for raising algae or catfish.
If the commonly held notion that the lower limit for energy rejection is dictated by man's natural environment, and that waste heat must be rejected to the environment, were understood as not truly limiting, a substantial improvement could be obtained in the thermal efficiency of power plants by substantially lowering the temperature at which heat is rejected to well below ambient temperatures. Though heat will, of course, be ultimately rejected to the environment, nothing prevents the use of several coupled thermodynamic cycles operating at different sink temperature. If this is accepted, it can be seen, for example, that by having an artificially provided sink of 100.degree. K, a power cycle can be operated with such a sink and with a source temperature of, for example, 1000.degree. K, thereby attaining a Carnot efficiency of 90 percent instead of the theoretical efficiency of 71 percent computed with the 298.degree. K temperature criterion.
If an amount of heat, -dQ.sub.h, is withdrawn from a thermal reservoir and supplied during the heating phase of an ideal reversible power cycle, the entropy of the system using the power cycle is increased by dQ.sub.h /T.sub.h. The overall entropy change of the system during the power cycle must be zero since it is a closed cycle. Therefore, the entropy of the system must also decrease by an amount, dQ.sub.h /T.sub.h at another phase of the cycle. This can be accomplished at a lower temperature, T.sub.c. The heat the system must reject to a thermal reservoir is equal to dQ.sub.h /T.sub.h, and since the heat rejected is at the lower temperature T.sub.c, the heat rejected is -dQ.sub.c which is less than dQ.sub.h and the difference is the work produced. The entropy change of the thermal reservoir is increased by the amount -dQ.sub.c /T.sub.c. The total entropy of the universe is the sum of the entropy change of the system and the reservoirs which, if totally reversible, would be zero. Any irreversibilities must make the entropy of the universe increase and never decrease. The second law of thermodynamics imposes no limit as to what specific sink may be used, as long as the entropy of the universe increases or remains the same. Thus, there is no reason why one cycle cannot operate at a sink of 100.degree. K, and an ancillary cycle utilized in combination therewith to provide such sink while such ancillary cycle is itself operating with a sink at 300.degree. K, and thus ultimately provides the point of heat rejection to the universe, making the entropy increase.
In sum, though it has not heretofore been apparent, an absorption cycle can operate with an environmental heat sink (operated at approximately ambient temperature), and yet provide a cold (sub-ambient temperature) sink as necessary to operate a primary cycle at an efficiency more nearly approaching the Carnot ideality.
Although most ancillary absorption cycles utilized in the manner described can only provide a sink temperature of as low as about 200.degree. K, the HDH cycle which is described in our copending application can provide a sink approaching the triple point of hydrogen, 50.degree. K. The absorption cycle thus provided can be very advantageously used, for example, with a direct cycle gas nuclear power plant using helium, argon or nitrogen, since the thermal efficiency of the primary cycle would be greatly improved by the lower heat sink, and the heat necessary to drive the absorption cycle by the operation of the hydride reactor would be readily available from lower temperature thermal energy (waste heat, etc.). Substantially all other types of existing power plants using conventional power cycles could also benefit greatly from the use of the auxiliary absorption cycle constituted by the HDH system, and near doubling of the power output using the same amount of fuel that is now used could be obtained in many instances.